1. Field of the Invention
This invention is directed to apparatus and methods for flooding a nuclear imaging device with radiation from a radiation source; for correcting for non-uniformities in count density in nuclear imaging sources; for providing uniform flooding of a nuclear imaging device by a standardized source (solid or liquid); and in one of its aspects is directed to such apparatus and methods for single photon emission computed tomography ("SPECT") and in another of its aspects is directed to such apparatus and methods for cobalt-57 sources.
2. Description of the Prior Art
In nuclear imaging, compensation can be made for various errors such as errors attributable to detection systems and items such as collimators. In compensating for these errors it is known to require a flood-field image of a standard radiation-emitting uniform source and to correct the images produced by a camera after it has been calibrated with the uniform source image. But in so calibrating nuclear imaging devices, a source with non-uniform radiation emission count density can cause errors for which compensation cannot be made. Especially in single photon emission computer tomography, source count density uniformity is a necessity; but sources, even certified standard sources, do exhibit varying degrees of count density non-uniformity. These source non-uniformities cannot be compensated for ("masked") as can deficiencies in the detection system (e.g. crystals or photo multiplier tubes) or deficiencies in a collimator.
Commercially available standard nuclear imaging sources, including solid sources and refillable liquid standards, exhibit some degree of non-uniformity. Refillable liquid standards such as liquid phantoms are subject to deformation, poor mixing, absence of retrospective verification, accidental contamination and substantial handling requirements. Increased handling required by refillable phantoms, such as a refillable liquid technetium-99m phantom, subjects technologists to increased radiation exposure.
Commercially available cobalt-57 solid sheet source standards generally fail to provide an adequate uniform count density. Such standards with a stated certified non-uniformity of .+-.1% may actually have a much higher non-uniformity if standard National Electrical Manufactures Association ("NEMA") procedures are applied, since certified non-uniformity is based on a sampling of counts from the surface of the standard rather than on a total surface radiography. Also, since non-uniformity is quantified from counts in only two smoothed pixels (the maximum and the minimum), the method is potentially dependent on small flaws.
One method proposed for compensating for source non-uniformity involves a tedious manual re-positioning of a source for mapping in multiple positions. See "Uniformity Correction for SPECT Using a Mapped Cobalt-57 Sheet Source," Volume 26, No. 4, The Journal of Nuclear Medicine, April 1985.
In accordance with .sctn.1.56 of 37 C.F.R., the following are disclosed:
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